Brother PT-530 Service Manual page 65

Powering-on sequence (See Figure 3.2-7.)
When the CPU is supplied with no power and is in the reset status, pressing the On/Off key turns
on Q3 so as to feed 5V power to the CPU. Getting started, the CPU first processes the INT0
interrupt and reads the input from the On/Off key. To avoid malfunctioning due to chattering
noises caused by accidental pressure to the key, the CPU reads the input level again after 10 ms
from the first receipt of the INT0. If the input is truly Low, the CPU interprets the On/Off key as
being pressed and starts setting up the entire system.
The CPU turns the POW signal (P86) High, making the base node input of transistor Q5 High so
as to keep supplying the Vcc power after the On/Off key is released.
Figure 3.2-7 Powering-on Sequence
Powering-off sequence (See Figure 3.2-8.)
Pressing the On/Off key when the CPU is running causes the same process as in the above
powering-on sequence until the CPU interprets the On/Off key as being pressed. Then, the CPU
turns the POW signal (P86) Low to shut down the Vcc.
Figure 3.2-8 Powering-off Sequence
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